Crossed-field electron injector for an electron accelerator

ABSTRACT

An electron injector wherein a stream of electrons is emitted from a gridded thermionic emitter into a region between two electrodes. An electric field alternating at a microwave frequency is applied between the electrodes, and the electron stream leaves the thermionic emitter parallel to this field. A steady magnetic field is applied across the region in a direction that is transverse to the electric field. The combined effects of the microwave electric field and the steady magnetic field on the electron stream are such that the stream becomes focused into a bunch that follows a quasi-cycloidal orbit in which the electrons strike a secondary emitter from which an amplified bunch of electrons is emitted to follow a similar orbit, to be further focused into a bunch and to strike another secondary emitter. This process is repeated until the desired intensity is achieved whereupon the final bunch of electrons is injected into an electron accelerator during the final orbit.

I United States Patent 1 1 3,593,058

[72} Inventor Harold A. Hogg 3,233,140 2/1966 Holshouser 313/103 X Cupertino. Calif. 3,388,282 6/1968 Hankin etalr .1 313/103 X [2]] Appl. Nu, 20,289 3,431,420 3/1969 Fisher 315/393 X [22] Wed 1970 Primary Examiner-Herman Karl Saalbach [45] Patented 1971 Assistant Exa ner-Saxfield Chatmo Jr [73] Assignee The United States of Americaas Amme 'g A Anderson represented by the United States Atomic Energy Commission ABSTRACT: An electron injector wherein a stream of electrons is emitted from a gridded thermionic emitter into a region between two electrodes. An electric field alternating at a [54] CROSSEDJIELD ELECTRON INJECTOR FOR AN microwave frequency is applied between the electrodes, and ELECTRON ACCELERATOR the electron stream leavesthe thermion c emitter parallel to 10 Claims, 3 Drawing Figs this field. steady magnetic field 11s applledacross the region in a direction that rs transverse to the electric field. The com- U-S-Cl bined effects of the microwave electric and the teady 315/541, 313/63 313/104 magnetic field on the electron stream are such that the stream [51] Int. Cl H013 7/46, becomcs f d into a bunch that follows a quasi-cycloidal j 19/80 orbit in which the electrons strike a secondary emitter from 0' Search an bunch of electrons is emittcd to follow a 105, 63; 315/541, similar orbit, to be further focused into a bunch and to strike another secondary emitter. This r'ocess is re eated until the [56,] Reerences cued desired intensity is achieved wh reupon the final bunch of UNlTED STATES PATENTS electrons is injected into an electron accelerator during the 2,925,522 2/1960 Kelliher 315/5.42 final orbit.

PATENTEH JUL 1 3 I971 Bl AS VOLTAGE SOURCE N? kw a a law INVENTOR. Harold A. Hogg' ATTORNEY.

CROSSED-FIELD ELECTRON INJECTOR FOR AN ELECTRON ACCELERATOR The invention disclosed herein was made under, or in, the course of Contract No. AT(04-3 )-40() with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to electron injectors for electron accelerators, and more particularly, it relates to a crossed-field injection gun.

In general an electron injector for a linear electron accelerator is used to generate electrons, form the electrons into a beam which is then aimed, focused and bunched 'before injection into the accelerator. Such an injector includes a number of parts including an electron gun, a prebuncher, a buncher, drift tubes and focusing devices including a large solenoid. All of these parts are coaxially arranged with the central axis of the accelerator and are used to form a pulsed and bunched beam that has a predetermined cross sectionand that enters the accelerating structure along the centralaxis of the structure. Such an injector has a large number of parts, is

complex and in operation requires a large number of high fering energies and currents. When using-a conventional injector for injecting electrons at some point along the length of the accelerator, the injector is mounted off the accelerator axis and the beam leaves the injector at an angle to the-axis. The beam therefore must be bent from its path alongthe injector axis to a path along the central axis of the acceleratorby means of large expensive high-energy bending magnets.

SUMMARY OF THE INVENTION In brief, the present invention relates to a crossed-field electron injector for injecting a beam of bunched electrons into the waveguide structure of an electron accelerator at any point along its length and comprises a housing that defines a microwave cavity having an aperture at one end that opens into the waveguide structure and is coaxially aligned with the. structure, a source of electrons at the other end of the cavity for introducing a stream of electrons into the cavity, a source of microwave energy for establishing a microwave field for accelerating the stream of electrons inalternate directions parallel to the axis of the waveguide structure to effect phasefocussing or bunching of the electron stream, a magnet for maintaining a steady magnetic field in the cavity in a direction that is transverse to the electric field and clear of the aperture, and a plurality of secondary emitters that are spaced at successive equal distances toward the aperture from the electron source whereby the stream of electrons is driven by the magnetic field into quasi-cycloidal orbits that have end points at the electron source, the aperture and the secondary emitters, which emitters, in conjunction with energy added to the .electron stream from the electric field, increase .thenumber of 65 rpleratnr Another object is to directly inject electrons at a secondary point along the length of a linear electron accelerator without using a large beam bending magnet and without placing equipment at the secondary point which would interfere with a primary electron beam from the beginning of the accelerator when it passes the secondary point of injection.

Another object is to improve beampunching in an electron injection system.

Another object is to reduce the total power requirements for an electron injection system.

Other objects and advantageous features of the invention will be apparent in a description of the specific embodiment thereof, given by way of example only, to enable one skilled in the art to readily practice the invention, and described hereinafter with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view, with portions removed and, broken away, of a crossed-field electron injector for a linear accelerator, according to the invention.

FIG. 2 is a cross-sectional view of a portion of the injector of FIG. 1 broken in the direction of arrows 2-2.

FIG. 3 is a full end view ofthe injector of FIG. 1 taken in the direction of arrows 3-3.

DESCRIPTION OF AN EMBODIMENT Referring to the drawing there is shown in FIG. 1 a perspective view of an electron injector 10 that is inserted along the,

length of a linear electron accelerator 12 in coaxial alignment. therewith for multiplying, bunching, and accelerating a beam; of electrons for injection into the accelerator. The injector 103. is comprised of a rectangular shaped housing 14 of electrically; conductive nonmagnetic material having a pair of electrically. conductivenonmagnetic bars 16 and 17 centrally mounted in opposing spaced-apart relationship on opposite internal sides of the housing. An electron emitter assembly 19 comprising a thermionic cathode, a heater and a grid is mounted in one end, I of the housing and extends through the housing and through one end of the bar 17 so that the grid surface is even with the; inner surface of the bar 17. The housing defines a cavity 15 in. which a steady magnetic field B is established in a direction indicated by the arrow B in FIG. 1 and the arrow end B into the, paper in FIG. 2. The steady magnetic field may conveniently. be established by means of an electromagnet comprised of a core 21 having left and right legs on which are wound coils 23. and 24 respectively. A pair of pole pieces 26 and 27 (FIG. 3) are mounted in the windowof the core 21 with a gap between: the pole pieces. The housing 14 is mounted in the gap so that r the steady magnetic field extends through the cavity 15. The, pole pieces 26 and 27 may be substantially identical in shape with one end bevelled so that the magnetic field B is clear of, an aperture or hole 37 in the bar 17. The hole 37' extends through the housing 14 and is the point of injection-of elec-- trons into the accelerator 12. The shape of the pole piece 27 ,j and therefore the shape of the magnetic field B, is indicated in, dotted outline in FIG. 2.

Microwave energy is coupled from a microwave source.(not .1 shown) to the cavity. 15 by means of a microwave coaxial line;

29 that is terminated with a coupling loop 31 for establishing a microwaveelectric field in the cavity in a direction that isr.

transverse to the steady magnetic field B. Conveniently, ther length of the cavity 15 may be made to be one-half wavelength, at the frequency of the microwave energy coupled to the cavity so as to establish an electric field that, is substantially:

uniform over the entire length of the bars 16 and 17. Theelec-. tric field alternates at themicrowave frequency to apply op-.

posing polarities to the bars 16 and 17 which tend to concen trate the field therebetween. The electrons from the assembly 19 are accelerated towards the bar 16 duringthe period, that,

the electric field is such that the voltage at the bar,,16 is positive with respectto the bar 17, while the orbitingeleetronsare accelerated towardsthebar 17 when the pplatityoftheelecz.

tric field is reversed. The electron current is controlled by a voltage applied between the thermionic cathode and the grid of the assembly 19. Motion of the electrons within the steady magnetic field B causes the electrons to follow a quasicycloidal orbit 33 which curves outward towards the bar 16 and then returns to the bar 17. At the point at which the electrons would impinge on the bar 17, a secondary emitter 35 is mounted. Multiplied secondary emission is obtained from the emitter 35 and additional multiplied secondary emission is obtained from successive equally spaced secondary emitters 35 and 35" also mounted on the bar 17. The resulting secondary electrons follow successive orbits 33', 33", and 33". The secondary emitters 35 may be made for example of magnesium oxide and are successively equally spaced apart along a straight line that includes the assembly 19 and hole 37. I

The individual electrons emitted from the assembly 19 follow different trajectories depending on the phase of the microwave electric field with respect to each electron. During the half cycle when the bar 16 is negative with respect to the bar 17 no electrons are emitted from the assembly 19. During the second half cycle, when the bar 16 is just becoming positive with respect to the bar 17, electrons are emitted and accelerated away from the bar 17, following along trajectory that comes close to the bar 16 and then loops back toward the bar 17 to strike the adjacent secondary emitter when the electric field reverses. Electrons emitted somewhat later in the second half cycle are subjected to a greater initial accelerating field which causes their trajectories to be more sharply curved by the steady magnetic field to thereby follow shorter, flatter paths to the adjacent secondary emitter than the electron emitted earlier in the cycle. Thus, the electrons emitted late in the second half cycle catch up with the electrons emitted earlier in the cycle. Electrons leaving very late (more than 40 to 60, depending on the parameters chosen) in the second half cycle begin to arrive back at the bar 17 with significant phase lag and to fall short of the secondary emitter. Thus, there is a strong selection and bunching of the emitted electrons. This phenomenon results in the electron bunches being increasingly sharpened in each succeeding orbit 33', 33", and 33". During the last orbit 33", the electron bunches leave the magnetic field along a straight path 39 at the point of entry into the aperture 37. The path 39 is coaxial with the central axis of the accelerating structure 12 so that the bunched electrons follow a path that is aligned with the central axis of the accelerator structure 12.

A hole 41 may also be provided in the bar 16 and housing 14 that is coaxially aligned with the hole 37 and the central axis of the accelerating structure 12. This permits a primary beam of electrons 43 to pass through the hole 41 and hole 37 without being a blocked or otherwise disturbed by the injector l0.

It may be found desirable to insert a short accelerating section 45 between the hole 37 and the main accelerating structure 12 for bringing the electrons from the injector to a velocity, typically near to the velocity of light, that is synchronous with the phase velocity of the accelerating wave traveling in the structure 12.

In operation, the microwave energy coupled to the cavity may be pulsed to supply a pulsed electron stream for injection into the accelerating structure 12. The beam 43 may be similarly pulsed but synchronized with the pulsing of the microwave energy so that pulses of the beam 43 pass through the injector 10 during a time period different from that of the pulses generated in the injector 10.

Generally, the primary beam 43 will be of such a high energy that it will pass through the injector 10 and the short accelerating section 45 without being significantly affected by the magnetic field B. However, it may be necessary to pass the primary beam through a compensator magnet before it enters the injector 10.

While an embodiment of the invention has been shown and described, further embodiments or combinations of those described herein, will be apparent to those skilled in the art without departing from the spirit of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A crossed-field electron injector for injecting a beam of bunched electrons into a waveguide structure ofa linear electron accelerator, comprising:

a. a linear electron accelerator including waveguide structure;

b. a housing defining a microwave cavity, said housing having a first aperture that opens into said waveguide structure and is coaxially aligned with the central axis of said structure;

c. an electron source for introducing a stream of electrons into the cavity, said source being an assembly comprised of a thermionic cathode and a grid for providing a stream of electrons into said housing, and a bias voltage source connected across said grid and cathode for controlling the intensity of electrons emitted from said cathode;

d. means for introducing microwave energy into said cavity for establishing a microwave field for accelerating said electrons in alternate directions for bunching said electrons;

means for maintaining a steady magnetic field in said cavity, said magnetic field being in a direction that is transverse to said electric field; and

. a plurality of discrete secondary emitters spaced at successive equal distances from said source toward said aperture, said magnetic field driving said electrons into quasicycloidal orbits that terminate at said successive equally spaced secondary emitters and said aperture, said secondary emitters in conjunction with the electric field causing an increase in the number of electrons in successive orbits, the electrons in the last orbit passing through said aperture for injection into said waveguide structure, the emitting surfaces of said emitters and said grid being positioned to lie in the same plane.

2. The injector of claim 1, wherein said housing is provided with a second aperture opposite said first aperture and coaxially aligned with the central axis of said waveguide structure for providing a clear path for electrons to move from one end of said accelerator to the other through said injector.

3. The injector of claim 1 wherein the cavity dimension in a direction that is transverse to the magnetic field and to the electric field is equal to one-half wavelength at the frequency of the microwave energy introduced into said cavity.

4. The injector of claim I, wherein said means for maintaining a steady magnetic field includes an electromagnet having a core having a gap, said housing being positioned in said gap, said core being shaped to establish said steady magnetic field in the cavity at a uniform intensity and in a volume that encompasses said orbits.

5. The injector of claim I, further including means for con centrating said electric field in the region of said orbital electron paths.

6. The injector of claim 5, wherein said concentrating means includes first and second electrically conductive bars mounted in said cavity on opposing walls of said housing and in electrical contact therewith, said electron source being mounted in said first bar, said first bar having a first opening through which electrons from said source are introduced into said cavity, said secondary emitters being mounted on said first bar, said first bar having a second opening on the end opposite said first opening, said second opening being coaxially aligned with said aperture in said housing and the central axis of said accelerating structure, said second bar being spaced from said first bar a distance slightly greater than the maximum radius of the orbital electron paths.

7. The injector of claim 7, wherein said source is a thermionic cathode, and further including a grid across said first opening and electrically connected to said bar, and a bias voltage source connected across said grid and cathode for controlling the intensity of electrons emitted from said cathode.

8. The injector of claim 7, wherein said means for maintaining a steady magnetic field includes an electromagnet having a pair of pole pieces with a gap therebetween, said housing linear accelerator for receiving electrons that pass through said first aperture for acceleruting said electrons to the synchronous velocity of said accelerating structure.

10. The injector of claim l wherein said discrete secondary emitters are formed of magnesium oxide. 

1. A crossed-field electron injector for injecting a beam of bunched electrons into a waveguide structure of a linear electron accelerator, comprising: a. a linear electron accelerator including waveguide structure; b. a housing defining a microwave cavity, said housing having a first aperture that opens into said waveguide structure and is coaxially aligned with the central axis of said structure; c. an electron source for introducing a stream of electrons into the cavity, said source being an assembly comprised of a thermionic cathode and a grid for providing a stream of electrons into said housing, and a bias voltage source connected across said grid and cathode for controlling the intensity of electrons emitted from said cathode; d. means for introducing microwave energy into said cavity for establishing a microwave field for accelerating said electrons in alternate directions for bunching said electrons; e. means for maintaining a steady magnetic field in said cavity, said magnetic field being in a direction that is transverse to said electric field; and f. a plurality of discrete secondary emitters spaced at successive equal distances from said source toward said aperture, said magnetic field driving said electrons into quasi-cycloidal orbits that terminate at said successive equally spaced secondary emitters and said aperture, said secondary emitters in conjunction with the electric field causing an increase in the number of electrons in successive orbits, the electrons in the last orbit passing through said aperture for injection into said waveguide structure, the emitting surfaces of said emitters and said grid being positioned to lie in the same plane.
 2. The injector of claim 1, wherein said housing is provided with a second aperture opposite said first aperture and coaxially aligned with the central axis of said waveguide structure for providing a clear path for electrons to move from one end of said accelerator to the other through said injector.
 3. The injector of claim 1 wherein the cavity dimension in a direction that is transverse to the magnetic field and to the electric field is equal to one-half wavelength at the frequency of the microwave energy introduced into said cavity.
 4. The injector of claim 1, wherein said means for maintaining a steady magnetic field includes an electromagnet having a core having a gap, said housing being positioned in said gap, said core being shaped to establish said steady magnetic field in the cavity at a uniform intensity and in a volume that encompasses said orbits.
 5. The injector of claim 1, further including means for concentrating said electric field in the region of said orbital electron paths.
 6. The injector of claim 5, wherein said concentrating means includes first and second electrically conductive bars mounted in said cavity on opposing walls of said housing and in electrical contact therewith, said electron source being mounted in said first bar, said first bar having a first opening through which electrons from said source are introduced into said cavity, said secondary emitters being mounted on said first bar, said first bar having a second opening on the end opposite said first opening, said second opening being coaxially aligned with said aperture in said housing and the central axis of said accelerating structure, said second bar being spaced from said first bar a distance slightly greater than the maximum radius of the orbital electron paths.
 7. The injector of claim 7, wherein said source is a thermionic cathode, and further including a grid across said first opening and electrically connected to said bar, and a bias voltage source connected across said grid and cathode for controlling the intensity of electrons emitted from said cathode.
 8. The injector of claim 7, wherein said means for maintaining a steady magnetic field includes an electromagnet having a pair of pole pieces with a gap therebetween, said housing being positioned in said gap to establish said steady magnetic field in said cavity, said pole pieces having a contour that establishes said magnetic field clear of said second opening in said first bar.
 9. The injector of claim 1, further including waveguide means coaxially aligned with the waveguide structure of said linear accelerator for receiving electrons that pass through said first aperture for accelerating said electrons to the synchronous velocity of said accelerating structure.
 10. The injector of claim 1, wherein said discrete secondary emitters are formed of magnesium oxide. 